This study presents a transient numerical investigation of heat transfer and airflow characteristics in a semi-cylindrical solar air heater using COMSOL Multiphysics. A three-dimensional conjugate heat transfer model was developed to evaluate the effect of daily solar-radiation variation on air temperature, local convective heat transfer, Reynolds number, and useful heat gain. The model includes transient solar heat flux on the absorber surface, heat loss from the outer collector surface by natural convection, no-slip wall conditions, and a free outlet boundary condition. The calculation was performed for a low inlet-air velocity of 0.01 m/s during the period from 08: 00 to 18: 00. For the adopted geometry, the hydraulic diameter was 0.611 m, and the Reynolds number was approximately (3.7–4.0) × 10 2 , indicating a laminar low-velocity regime. The maximum absorber-region temperature reached about 371–374 K during peak solar irradiation, while the maximum air temperature at the control points was about 338–340 K. The maximum local convective heat transfer coefficient and useful heat gain were approximately 0.176 W/(m 2 ·K) and 2.3–2.4 W, respectively. The results show that the semi-cylindrical geometry forms a stable temperature field under low-velocity conditions. However, the useful heat output remains limited because of the small air mass flow rate. Therefore, the developed model should be considered as a baseline numerical tool for further optimization at higher airflow rates and for future experimental validation.
Mirzayev et al. (Wed,) studied this question.